Answer: 21 goodluck
Step-by-step explanation:

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I got this:
Tennis court is a rectangle
(Rectangle - 2 sides the same length and the other 2 sides the same. So 2 pairs of equal length sides)
If one is 27 the other has got to be 27 as well - 27+27= 54
210 - 54 = 156
156 / 2 = the other "pair"
Hope that helps and correct me if I'm wrong!
For this case we can make the following rule of three:
21 ------> 70%
x --------> 100%
Clearing x we have:
x = (100/70) * (21)
Rewriting we have:
x = (10/7) * (21)
x = 30
Thus,
21 is 70% of 30
Answer:
21 is 70% of 30
Here is the graph with your line in it.